Energy storage is ubiquitous in our daily lives. For example, various batteries can be found in mobile electronic devices, automobiles, or utility-scale backup systems. Also, energy storage is becoming more important as our energy composition transforms into a renewable-focused structure, at least in part because of the intermittent nature of renewable energy sources.
Electrochemical cells are energy storage devices in which energy from reduction-oxidation (redox) chemical reactions are converted to electrical energy. An electrochemical cell generally includes a cathode, an anode, and the electrolyte. Reduction and oxidation reactions occur at the cathode and anode, respectively, while charged ions are transported through the electrolyte to generate an electrical current. In a “primary cell” (e.g., a non-rechargeable battery), the electrochemical reactions are irreversible. As the stored electrochemical energy is exhausted, the battery can no longer be used. Redox reactions in a “secondary cell” are reversible, and such batteries can be referred to generally as “rechargeable.”
The lead acid battery, invented by Gaston Planté in 1859, is generally the oldest rechargeable battery technology. Active electrochemical materials in the lead acid battery include lead dioxide (PbO2) and lead (Pb), which are generally pasted onto lead gird current collectors to form the positive and negative electrodes, respectively, in generally-available lead acid batteries. The electrodes are arranged electrically in parallel and are immersed into sulfuric acid (H2SO4) electrolyte to form a battery assembly. Such generally-available lead acid batteries are robust, economical, and relatively safe to operate. Therefore, lead acid batteries have found a wide range of consumer and industrial applications, and remain a commonly-used battery chemistry.